FR2741827A1 - Fabrication of tank from steel panels coated with aluminium@ - Google Patents

Abstract

The fabrication of a fuel tank made up of steel panels coated with aluminium, comprises: a) overlapping the respective edges (2a, 3a) of a pair of envelope elements (2, 3) made of steel panels coated with aluminium; and b) welding these edges (2a, 3a) to each other by a laser or electron beam to form the fuel tank. One (3a) of the edges (2a, 3a) has a stepped zone (d) leaving a space between the edges (2a, 3a). The laser beam is applied on the edges (2a, 3a) close to this space in order to evacuate unwanted constituents, such as aluminium, from the weld zone to the inside of the space, and thus the amount of aluminium in the weld zone is reduced to improve the strength of the weld. The welded product and the fuel tank produced by this method are also claimed in which the edges are laser or electron beam welded to form a weld zone containing at most 0.65% by weight of aluminium.

Description

 The present invention relates to an improvement in beam welding technology of aluminum-coated steel panels of an assembly such as a fuel tank intended for use in motor vehicles.

 Fuel tanks for use in motor vehicles such as motor scooters or the like generally consist of steel panels coated with tin-lead or panels of steel coated with nickel-zinc to make their Corrosion resistant interior surfaces. Since certain legal restrictions are imposed on the use of lead coating layers for environmental protection purposes, however, it has become apparent in the art of applications for fuel tanks made of other materials. Such a fuel tank consists of steel panels each having layers of fusible aluminum coating on its two faces, the steel panels having marginal edges superimposed and welded to each other. Aluminum cladding on both surfaces of the steel panels, the fuel tank of this structure is corrosion resistant on its interior and exterior surfaces.

Generally, the steel panels intended to make fuel tanks have so far been welded by electrical resistance welding, in a continuous line. If electrical resistance welding, in a continuous line, is used to weld steel panels each having layers of coating of fusible aluminum on its two surfaces, then this has the following disadvantages
the welding electrodes of the welding machine must be frequently cleaned and reshaped because the coated aluminum segments tend to get fixed on the welding electrodes during welding.

 Since the aluminum cladding layers are highly conductive of heat and electricity, the steel panels cannot be welded unless they are supplied with strong electric current which is 1.5 to 2, 0 times the electrical current required for ordinary steel panels.

 Because the amount of heat applied to the steel panels is large during welding and since the surface layers of the steel panels are destroyed in a large area by the applied heat, surface treatment after welding is necessary specifically on the inner surface of the fuel tank, to resist corrosion.

 It is therefore an object of the present invention to provide a beam welding process for aluminum coated steel panels so as to eliminate a subsequent preventive treatment against corrosion on its interior surfaces. and exterior, to increase the resistance of the welds, to prevent the coated aluminum segments from fixing on the welding electrodes, and to minimize the area in which the surface layers of the steel panels are destroyed.

 According to the present invention, there is provided a method for manufacturing a fuel tank made of aluminum-coated steel panels, comprising the steps consisting in superimposing respective flanges of two elements forming an envelope of aluminum-coated steel panels, and welding the flanges to each other by a beam, thereby forming a fuel tank.

 The beam can be applied to the flanges from the beam welder approximately perpendicular to the flanges while the beam welder is moved along a peripheral edge of the flanges. Alternatively, the beam can be applied to the outer peripheral edges of the flanges from a beam welding machine roughly parallel to the flanges while the beam welding machine is moved along the outer peripheral edges of the flanges In another variant, one of the flanges can project outwards over a greater distance than the other flange, and the beam can be applied obliquely to the outer peripheral edge of the other flange from a beam welding machine while the beam welding machine is moved along the outer peripheral edges of the flanges.

 According to the present invention, there is also provided a method of laser beam welding of aluminum coated steel panels, comprising the steps of superimposing the edges of two aluminum coated steel panels, a space being defined between them. , and welding the flanges to each other using a beam applied close to the space while discharging into space unwanted components from the welded area.

 One of the aluminum-coated steel panels may have a step-shaped area spaced from the other of the aluminum-coated steel panels, thereby defining the space between the step-shaped area and the other aluminum coated steel panels. The aluminum coated steel panels can be welded in the form of a container such as a fuel tank, and the space can be opened inwardly into the container or outward from the container .

 According to the present invention, there is further provided a welded construction produced by beam welding, comprising two steel panels coated with aluminum welded to each other by a beam, the steel panels coated with aluminum. being connected to each other by a welded zone which comprises at most 0.65% by weight of aluminum.

 According to the present invention, there is also provided a fuel tank supplied by beam welding comprising two casing elements made of aluminum coated steel panels having respective flanges overlapping each other and welded together. to the other by a bundle, the flanges being connected to each other by a welded zone which comprises at most 0.65% by weight of aluminum.

The objects, details and advantages of the present invention, as well as others, will appear on reading the detailed description which follows of preferred embodiments thereof, made with reference to the accompanying drawings, in which
- Figure 1 is a perspective view of a fuel tank made of aluminum-coated steel panels produced by the manufacturing process according to the present invention
- Figures 2A to 2C are partial sectional views showing various welding methods according to the present invention
- Figure 3 is a partial sectional view of a fuel tank having welded panels of aluminum-coated steel, one of which has a step-like area to create an open space inward
- Figure 4 is a partial sectional view of a fuel tank having welded panels of aluminum-coated steel, one of which has a step-like area to create a space open to the outside
- Figure 5 is a view showing the manner in which test pieces are welded;
- Figure 6 is an enlarged sectional view of a welded area of the test pieces having a space defined between them
- Figure 7 is an enlarged sectional view of a welded area of test pieces having no defined space therebetween
- Figure 8 is a graph showing the results of a tensile test carried out on test pieces to check the resistance of the bond
- Figure 9 is a graph showing the relationship between the amount of aluminum in the welded area and the tensile strength stresses;
- Figure 10A is a front view of test pieces used in the tensile test; and
- Figure 10B is a side view of the test pieces shown in Figure 10A.

 As shown in Figure 1, a fuel tank 1 made of aluminum-coated steel panels produced by a manufacturing process according to the present invention comprises a pair of upper and lower casing elements 2, 3 which are made press from aluminum coated steel panels. The upper and lower envelope elements 2, 3 have respective flanges 2a, 2b superimposed and welded to each other, all along their peripheral length, in the form of a hermetically sealed welded zone welded by laser beam. A fuel inlet arrangement 4 and various other small accessories such as uprights are connected to the upper and lower casing elements 2, 3 by welding or brazing by laser beam.

 Each of the steel panels coated with aluminum comprises a steel panel made of SAlE 60 steel or of SA1E60Y steel, which is obtained by chromating SAlE60 steel, the steel panel being coated with aluminum on its two surfaces. SAlE60 steel or SA1E60Y steel comprises, in addition to carbon, silicon and manganese which are necessary for refining purposes, and also impurities consisting of phosphorus, sulfide, etc.

 Iron has a melting point of 1540 "C and a boiling point of 2750" C, and aluminum has a melting point of 660 "C and a boiling point of 2470" C.

 The flanges 2a, 3a of the upper and lower envelope elements 2, 3 can be superimposed and welded by any method among various welding methods shown in FIGS. 2A to 2C.

 According to the welding process shown in FIG. 2A, a laser beam, indicated by the arrow, is applied to the edges 2a, 3a from a laser beam welding machine roughly perpendicular to the edges 2a, 3a while the laser beam welding machine is moved along an outer peripheral edge of the flanges 2a, 3a. This welding process is very resistant to poor alignment in the undesirable position of the laser beam on the edges 2a, 3a. However, the upper surface of the weld bead located on the welded edges 2a, 3a, will be coated with a corrosion-resistant layer to keep a desired level of aesthetic appearance.

 According to the welding process shown in FIG. 2B, a laser beam, indicated by the arrow, is applied to the outer peripheral edges of the flanges 2a, 3a from a laser beam welding machine roughly parallel to the flanges 2a , 3a while the laser beam welding machine is moved all along the outer peripheral edges of the flanges 2a, 3a. This welding process does not require that the weld bead be coated with a corrosion-resistant layer although its ability to prevent misalignment in the undesirable position of the laser beam on the welded flanges 2a, 3a is relatively weak.

 According to the welding process shown in FIG. 2C, one of the flanges 2a, 3a, for example, the flange 2a, projects over a greater distance towards the outside than the flange 3a, and a laser beam, indicated by the arrow , is applied obliquely to the outer peripheral edge of the shorter edge 3a from a laser beam welding machine while the laser beam welding machine is moved all along the outer peripheral edges of the edges 2a, 3a . This welding process is absolutely resistant to misalignment in the undesirable position of the laser beam on the edges 2a, 3a. However, a corrosion resistant layer will be coated on the upper surface of the weld bead on the welded edges 2a, 3a to maintain a desired level of aesthetic appearance.

 When the welding method shown in FIG. 2A is used, if the flanges 2a, 3a are kept in intimate contact with each other while being welded by laser beam, then the resistance of the connection of the flanges 2a, 3a is greatly reduced, and is a function of the speed at which the laser beam welding machine moves.

 Generally, when zinc coated steel panels are welded to each other by a laser beam, since the zinc coating layers have a low boiling point, gas bubbles are produced in the welded area due to zinc vapor. It has been proposed to provide a space between the zinc-coated steel panels for discharging such zinc vapor (see, for example, Japanese patent application No. 4-13,077).

 When aluminum coated steel panels are welded by a laser beam, no gas bubbles are produced in the welded area by aluminum vapor since the aluminum coating layers have a point of high boil. It has been found that the resistance of the welded area of aluminum coated steel panels varies greatly depending on the metallic composition of the welded area.

 The present inventors conducted a tensile test on T test pieces (see Figure 5) each having the shape of a steel panel coated with aluminum.

The test pieces T are spaced at different spaces or different distances from each other by spacers S, and are welded by laser beam to each other by a laser beam welding machine then that the laser beam welding machine is moved at different speeds. After this, the tensile test is carried out on the welded test pieces T to check their bond strength. The results of the tensile tests are shown in Figure 8.

 When there is no space between the test pieces T, the pressure imposed on the test pieces T is modified by clamps C (see FIG. 5). When there is a space between the test pieces T, the space existing between the test pieces T is modified by variation of the height of the spacers S. In FIG. 8, the horizontal axis represents the space ( mm) existing between 2 the test pieces T and also the pressure (kg / cm2) applied to the test pieces T if there is no space between them, and the vertical axis represents the stresses 2 of tear fracture (kgf / mm). The marks A, I represent samples which have been welded by the laser beam welding machine which moves at a speed equivalent to the speed used by a continuous line welding machine. Specifically, the mark A represents samples which are welded by the laser beam welding machine which moves at a speed of 3.5 m / min., while the laser beam is focused at a point located 4 mm above the workpiece upper test T, and the mark I represents samples which have been welded by the laser beam welding machine which travels at a speed of 4.0 m / min. while the laser beam is focused at a point on the upper surface of the test piece T. The mark represents samples which are welded by the laser beam welding machine which moves at a markedly higher speed low 1.5 m / min. while the laser beam is focused at a point 4 mm above the upper test piece T.

 As shown in FIGS. 10A and 10B, the test pieces T have an L shape, and the shorter arms of the test pieces T have been superimposed and welded.

In the tensile test, the longer arms of the test pieces T were pulled in opposite directions indicated by the arrows in Figure 10B.

When there is no space between the test pieces T and the pressure applied to the test pieces 2 is 5 kg / cm 2, the fracture stresses of the samples A are in the range from 5 to 10 kgf / mm2, and those of samples I are lower (not shown). Consequently, the stresses of fracture by pulling out of the samples A, are very low. When there is no space between the test pieces T and the pressure applied 2 to the test pieces T is 10 kg / cm2, the stresses of fracture by pulling out the samples are
2 located in the range from 22 to 27 kgf / mm2, and therefore are relatively large. When there is a space between the test pieces T, the samples presented significant stress fracture fractures, close to the stresses parent metal fracture regardless of the speed at which the laser beam welding machines move and the position in which the laser beam is focused.

 The changes in the fracture stresses as a function of the space existing between the test pieces T are clearly seen in the microstructure diagrams of the welded zones represented in FIGS. 6 and 7. When there is no space between the test pieces T as shown in Figure 7, unwanted components "g" such as aluminum, etc.

mainly separated from the aluminum cladding layers located on the overlapping surfaces cannot find their way outward, and are trapped and solidified in a welded "y" area. When there is a space between the test pieces T as shown in Figure 6, these unwanted components "g" are expelled from a welded area "y" to the interior of the space because of the difference in solidification temperature, and are solidified in space.

 Based on the above test data, it has been found that when there is space between aluminum clad steel panels which are welded, sufficient bonding force is obtained between the steel panels coated aluminum welded, and when there is no gap between the aluminum coated steel panels which are welded, a desired bonding force cannot be obtained between the coated aluminum panels welded aluminum unless the laser beam welding machine moves at a sufficiently low speed, that is to say that the quantity of thermal energy of the laser beam applied per unit area is large enough.

 The inventors have considered that the aluminum contained in the welded zone "y" may have an influence on the tensile strength of the welded zone "y" and have verified the manner in which the amount of aluminum contained in the central zone of the welded area is related to the stresses of tear fracture. Figure 9 represents the relation measured between the quantity of aluminum contained in the central zone "a" (see figure 6) of the welded zone "y" and the stresses of fracture by tearing. In reality, figure 9 represents the results a tensile test, similar to the tensile test described above with reference to Figure 8, carried out on test pieces T to check the bond strength after the test pieces T have been welded by laser beam at different spaces between them under the same conditions as those described above with reference to FIG. 8. In FIG. 9, the horizontal axis represents the quantity of aluminum contained in the welded zone, and the vertical axis represents fracture fracture stresses.

 It can be understood from FIG. 9 that if the quantity of aluminum contained in the welded zone "y" is 0.65% by weight or less, then the stresses of fracture by tearing off of the welded test pieces T are the same as those of the parent metal (27 to 28 kgf / mm2), and if the quantity of aluminum contained in the welded zone "y" is greater than 0.65% by weight, then the stresses of tear fracture T test pieces are weaker than those of the parent metal. Consequently, the aluminum contained in the welded area "y" is related to the tensile strength so that the bonding strength of the welded area "y" increases when the amount of aluminum contained in the welded area "y" decreases. It can also be seen from FIGS. 8 and 9 that in order to reduce the quantity of aluminum contained in the welded zone "y" to obtain a desired bonding resistance, it is effective to provide a space at the welded area and increase the amount of thermal energy of the laser beam applied per unit area as well as lower the speed at which the laser beam welding machine moves. Therefore, it is desirable that the amount of aluminum contained in the welded area "y" is 0.65% by weight or less to obtain a desired level of bond strength.

 Figures 3 and 4 show a fragmentary view of fuel tanks, respectively, having aluminum-coated steel panels which are welded according to the welding process shown in Figure 2A.

 In FIG. 3, the fuel tank comprises upper and lower casing elements 2, 3 having respective flanges 2a, 3a superimposed and welded to one another. The lower edge 3a has a tread-shaped zone "d" extending along an inner peripheral edge thereof and is moved down from the upper edge 2a, creating a space opening towards inside in the fuel tank. The upper and lower edges 2a, 3a are welded to each other by a laser beam, indicated by the arrow, applied downwards on the upper edge 2a above the step-shaped zone "d" . Since the upper and lower edges 2a, 3a are kept in intimate contact with each other along their outer edges, the upper and lower edges 2a, 3a can easily be clamped together by pliers.

 In FIG. 4, the fuel tank comprises upper and lower casing elements 2, 3 having respective flanges 2a, 3a superimposed and welded to each other. The lower rim 3a has a step-shaped zone "d" extending along an outer peripheral edge thereof and is moved down from the upper rim 2a, creating a space opening towards the outside of the Fuel tank. The upper and lower edges 2a, 3a are welded to each other by a laser beam, indicated by the arrow, applied downwards on the upper edge 2a above the step-shaped zone "d". As long as the upper and lower edges 2a, 3a are kept out of contact with each other along their outer edges, they cannot be clamped together by pliers along their outer edges unless spacers are not inserted between the upper and lower edges 2a, 3a. However, the fuel tank shown in Figure 4 offers an advantage in that unwanted components such as aluminum separated from the welded "y" area (see Figure 6) cannot enter the fuel tank. fuel, but can easily be evacuated to the outside through the open space to the outside existing between the upper and lower flanges 2a, 3a.

 As described above, space can easily be provided between the flanges 2a, 3a by providing the step-shaped area "d" on one of the flanges 2a, 3a.

 Since the fuel tank according to the present invention is made of aluminum coated steel panels welded together, the welded area formed between the aluminum coated steel panels is protected by a reactive action of corrosion resistance of aluminum, making the bottom surface of the fuel tank highly resistant to corrosion.

Aluminum-coated steel panels do not need to be further treated by coating them with layers.

 Because the aluminum coated steel panels are welded by a laser beam having a small beam point, any area in which the surface layers of the aluminum coated steel panels are destroyed by the laser beam, is minimized.

Laser beam welding allows aluminum coated steel panels to be welded at a high speed since they can be heated quickly at a localized point.

 Laser beam welding does not require any welding electrode intended to be brought into contact with aluminum coated steel panels. Therefore, it is free from any problem of aluminum segment fixed on the welding electrodes.

 The space formed between the aluminum coated steel panels allows unwanted components to be vented from the welded area. Consequently, these unwanted components discharged from the welded area do not get inside the molten metal located in the welded area, which keeps a desired level of bond strength.

 While laser beam welding has been described in the above embodiments, the principles of the present invention can also be applied to aluminum coated steel panels welded by electron beam welding.

 Although it has been described what is currently considered to be the preferred embodiments of the present invention, it will be understood that the present invention can be embodied in other specific forms without departing from the essential features thereof. The present embodiments should therefore be considered in all their aspects as representative, and not as restrictive.

The scope of the present invention is indicated by the appended claims rather than by the foregoing description.

Claims (10)

 1. Method for manufacturing a fuel tank (1) made of aluminum-coated steel panels, comprising the steps consisting in:  covering the respective flanges (2a, 3a) with a pair of envelope elements (2, 3) made of aluminum-coated steel panels; and  welding said flanges (2a, 3a) to each other using a beam, thereby forming a fuel tank (1).  2. Method according to claim 1, characterized in that said beam is applied to said flanges (2a, 3a) from a beam welding machine, roughly perpendicular to the edges, while the beam welding machine is moved along a peripheral edge of the edges (2a, 3a).  3. Method according to claim 1, characterized in that said beam is applied to the outer peripheral edges of the flanges (2a, 3a) from a beam welding machine roughly parallel to the edges while the welding machine by beam is moved along the outer peripheral edges of the edges (2a, 3a).  4. Method according to claim 1, characterized in that one (2a) of said flanges (2a, 3a) protrudes a greater distance towards the outside than the other flange (3a), and said beam is applied obliquely on the outer peripheral edge of said other flange (3a) from a beam welding machine while the beam welding machine is moved along the outer peripheral edges of the flanges (2a, 3a).  5. A method of laser beam welding of steel panels coated with aluminum, characterized in that it comprises the steps consisting in  superimposing the respective flanges (2a, 3a) of a pair of panels (2, 3) of steel coated with aluminum, a space being defined between them; and  weld said flanges to each other using a beam applied close to said space while removing unwanted components (g) from a welded area (y) to the inside of said space.  6. Method according to claim 5, characterized in that one (3) of said panels (2, 3) of steel coated with aluminum has a step-shaped zone (d) spaced from the other (2) aluminum-coated steel panels, therefore defining said space between said step-like area (d) and said other of aluminum-coated steel panels.  7. The method of claim 6, for manufacturing a container made of welded panels of aluminum-coated steel, characterized in that said space opens inwardly into the container.  8. Method according to claim 6, intended for manufacturing a container made of welded panels of steel coated with aluminum, characterized in that said space opens towards the outside of the container.  9. Welded construction produced by beam welding, characterized in that it comprises  a pair of aluminum coated steel panels (2, 3) welded to each other by a bundle, the aluminum coated steel panels being connected to each other by a welded zone which contains at most 0.65% by weight of aluminum.  10. Fuel tank (1) produced by beam welding, characterized in that it comprises  a pair of casing elements (2, 3) made of aluminum coated steel panels having respective flanges (2a, 3a) superimposed and welded to each other by beam, the flanges being connected 'to each other by a welded zone which contains at most 0.65% by weight of aluminum.